The production method of a ceramic circuit substrate, which may be called simply “circuit substrate” below, by brazing is disclosed, for example, in JP 2003-110222 A. The production method of a circuit substrate described in JP 2003-110222 A comprises bonding metal plates to at least one surface of a ceramic substrate via a brazing material; coating predetermined portions of the metal plates with a resist to etch unnecessary portions of the metal plates, thereby forming metal circuits; removing a residual brazing material and reaction products of the brazing material and the ceramic substrate while keeping the resist; and then removing the resist to form circuit patterns.
More specifically, the production method of a circuit substrate disclosed in JP 2003-110222 A comprises the steps of applying a brazing material paste comprising metal powder containing Ag, Cu, etc. in a predetermined composition, an organic binder and an organic solvent to both surfaces of the ceramic substrate by screen printing; setting Cu plates in contact with the brazing material; heating them at a predetermined temperature to bond the Cu plates to the ceramic substrate via the brazing material; coating predetermined portions of the metal plates with a UV-curing, alkali-soluble resist; removing unnecessary portions of the Cu plates with an etching solution comprising copper chloride, hydrogen peroxide water and hydrochloric acid to form metal circuits; immersing the product in a 30-% aqueous sodium thiosulfate solution while keeping the above resist; immersing the product in a brazing-material-removing liquid comprising EDTA and hydrogen peroxide water for a predetermined period of time to remove a residual brazing material and reaction products of the brazing material and the ceramic substrate; and then removing the resist.
As described above, in the production of a circuit substrate by a brazing method, a method comprising the steps of applying a brazing material paste comprising brazing material powder and an organic binder to a ceramic substrate, setting metal plates in contact with the brazing material paste to form a bonded body comprising the ceramic substrate, the brazing material paste and the metal plates, heating the bonded body at a brazing-material-melting temperature in vacuum or in non-oxidizing state to bond the metal plates to the ceramic substrate via the brazing material is widely used. The organic binder contained in the brazing material paste is decomposed and gasified in the heating process of the bonded body, so that it is removed from the brazing material paste. However, part of the gasified organic binder is likely deposited onto exposed surfaces of the ceramic substrate, which are not covered with the metal plates, forming carbon-based adhesive deposits. In the heating of the bonded body, deposits from a heating furnace atmosphere, etc. may also be formed on the bonded body.
Such deposits attached to the ceramic substrate surface are black or gray, evidently different from the color of the ceramic substrate, thereby being treated as defects in appearances. Also, because many of the deposits are highly conductive, sufficient electric insulation is unlikely secured between two metal plates constituting the metal circuit substrate, when large amounts of deposits are attached to the ceramic substrate surfaces exposed in a gap between the two metal plates, providing the circuit substrate with insulation resistance defects. It is extremely difficult to remove the insulation-resistance-decreasing deposits strongly attached to the ceramic substrate surface, by the etching solution or brazing-material-removing liquid disclosed in JP 2003-110222 A. When the ceramic circuit substrate is immersed in these solutions for a long period of time to remove the deposits completely, the resist and the metal plates are eroded, resulting in damaged metal plates. When the ceramic substrate is composed of a silicon nitride ceramic, the substrate tends to have large surface roughness due to the influence of columnar silicon nitride grains, making the removal of deposits further difficult.